1. Field of the Invention
The present invention is directed to electronics. The present invention further relates to memory for a wireless communications device.
2. Description of Related Art
On a complexity per cubic inch scale, cellular telephones are some of the most intricate devices people use on a daily basis. Modem digital cellular telephones can process millions of calculations per second in order to compress and decompress the voice stream. FIG. 7 shows a block diagram of a conventional portable radio telephone 10 (a.k.a. cell phone, cellular phone, mobile phone, wireless phone, etc.). The cell phone 10 includes an antenna 12 coupled to a transmitter 14 a receiver 16, as well as, a microphone 18, speaker 20, keypad 22, and display 24. The cell phone 10 also includes a power supply 26 and a central processing unit (CPU) 28, which may be an embedded controller, conventional microprocessor, or the like. In addition, the cell phone 10 includes integrated, NOR-type flash memory 30. NOR-type flash memory will be defined below with reference to FIG. 8.
Flash memory, in general, is a type of electrically erasable programmable read only memory (EEPROM). Like EEPROM, flash memory is nonvolatile and thus can maintain its contents even without power. However, flash memory is not standard EEPROM. Standard EEPROMs are differentiated from flash memory because they can be erased and reprogrammed on an individual byte or word basis while flash memory can be programmed on a byte or word basis, but must be erased on a block basis. Although standard EEPROMs may appear to be more versatile, their functionality requires two transistors to hold one bit of data. In flash memory, only one transistor is required to hold one bit of data, which results in a lower cost per bit.
In flash memory, the basic storage cell is a MOSFET transistor with a floating gate that is electrically isolated between the control gate and the MOSFET channel. A cell is programmed by transferring an electric charge to the floating gate (e.g. by channel hot electron injection) and erased by removing the charge (e.g. by Fowler-Nordheim tunneling). The floating gate is in close physical proximity to the MOSFET channel so that even a small electric charge on the floating gate has an easily detectable effect on the electrical behavior of the transistor. By applying appropriate signals to the MOSFET terminals and measuring the change in the transistor behavior, it is possible to determine whether or not there is an electric charge on the floating gate. The charge on the gate is then compared with a threshold value and each cell is characterized as “1” or “0”.
Flash memory comes in two primary varieties, NOR-type flash and NAND-type flash. While the general memory storage transistor is the same for all flash memory, it is the interconnection of the memory cells that differentiates the designs. FIG. 8A shows the conventional NOR-type cell architecture and FIG. 8B shows the conventional NAND-type cell architecture. In NOR-type flash memory, the memory cell transistors are connected to the bit lines in a parallel configuration, while in NAND-type flash memory, the memory cell transistors are connected to the bit lines in series. This is why NOR-type flash is sometimes referred to as “parallel flash” and NAND-type flash is referred to as “serial flash.”
Traditionally, cell phone CPUs have needed only a small amount of integrated NOR-type flash memory to operate. However, as cell phones have become more complex, offering more features and more services, memory requirements have steadily increased. Unfortunately, contrary to the normal trend for memory devices, such as dynamic random access memory (DRAM), NOR-type flash memory pricing has increased at a rate beyond the pro rata increases in capacity for such devices.
By far, NOR-type flash memory is the dominant type of flash memory in the market today. NOR-type flash memory has fast random read speeds, making the NOR architecture suitable for the fast random access required for program execution. In addition, the interface to NOR-type flash memory is designed for direct memory access, having separate address, data, and control lines. This enables a direct connection to microprocessors and permits direct execution of program code. For these reasons, manufactures have chosen to utilize NOR-type flash memory in prior art cell phones to execute program code, applications and the like. NAND-type flash memory, on the other hand, has relatively slow random read speeds compared to NOR-type flash memory, but fast erase and program speeds. In addition, the interface to NAND-type flash memory is indirect so there are no dedicated address lines or data lines, just control lines and I/O ports. This makes NAND-type flash memory relatively unsuitable for direct CPU access and execution of program code, but ideal as a mass storage device.
The current telecommunications market, especially the market for cell phones is extremely competitive. Consumers have many choices. While cell phone feature sets and services can distinguish one phone from the next, price is still an ultimate concern for a vast majority of users. To remain competitive in this market, manufacturers are constantly striving to build lower cost cell phones. Increases in NOR-type flash memory pricing, however, have made it difficult for manufacturers to achieve this goal. Any economies that can be gained in cell phone manufacture would be significant in this tight market.